Ecotoxicology of Manufactured Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 295

selenium, cadmium -selenium and cadmium-tellurium (Hardman, 2006). One of
the most valuable properties of QDs is their intense fl uorescence, which renders
them useful for biomedical imaging. Fluorescent QDs can be conjugated with bio-
active moieties (e.g. antibodies, receptor ligands) to target specifi c biologic cellular
structures, such as DNA and cell membrane receptors (Hardman, 2006).
Concern has been raised over the potential cytotoxicity of QDs owing to the
toxic nature of the parent materials (Hardman, 2006). For instance, the cytotoxicity
of bulk cadmium selenide (CdSe) is well documented (Derfus et al. , 2004 ). Toxic
metal ions such as Cd 2+ may be released upon oxidation of the quantum dot. Zinc
sulfi de coatings, which are often applied to QDs, may also oxidise in oxygenated
environments, leading to the release of zinc ions and then the oxidation products
of the QD core. QD oxidation and release Cd 2+ may also be enhanced by illumina-
tion with UV light (Derfus et al. , 2004 ).
A number of studies have investigated the cytotoxicity of QDs (Derfus et al. ,
2004 ; Kirchner et al. , 2005 ; Hardman, 2006 ). Derfus et al. (2004) have reported that
CdSe QDs release Cd 2+ ions in aqueous solution and that the concentration of the
Cd 2+ ions directly correlates with cytotoxic effects to liver hepatocytes. Surface
coatings such as zinc sulfi de and bovine serum albumin were shown to signifi cantly
reduce, but not eliminate, cytotoxicity.
To date, no ectoxicity information is available for QDs. Given the high toxicity
of cadmium to aquatic and terrestrial organisms it is likely that QDs will exhibit
substantial toxicity to test organisms.

7.5.7 Iron

Nanoscale Fe(0) iron particles (commonly referred to as zero-valent iron) repre-
sent a new generation of environmental remediation technologies. Nanoscale zero-
valent iron particles have large surface areas and readily undergo oxidation. They
are effective for the dechlorination and detoxifi cation of a wide variety of common
environmental contaminants, such as chlorinated organic solvents, organochlorine
pesticides and polychlorinated biphenyls (PCBs) (Zhang, 2003). Zero-valent iron
nanoparticles have been used at more than 20 sites for the in situ remediation of
groundwater contaminants in pilot or full scale operations (Wiesner et al. , 2006 ).
The particles can be injected directly into groundwater or used to detoxify con-
taminated water in above-ground tanks. These particles are of potential ecotoxi-
cological interest (particularly to soil organisms) given their potential release into
the environment. Natural iron hydroxyoxide colloids are ubiquitous in water and
soil systems; however, the high reactivity and oxidative potential of the zero-valent
iron nanoparticles may pose some issues of toxicity.
Oberdorster et al. (2006b) investigated the toxicity of iron nanoparticles with
an average particle size of 70 nm to the water fl ea ( Daphnia magna ) using
standard US EPA test methodology. It was reported that the particles were com-
posed of an iron oxide shell and an elemental iron core. The 48-h LC 50 for Daphnia
magna was 55 mg/l, which was approximately the same as that for bulk iron.

Further studies are clearly needed to evaluate the toxicity of zero-valent iron,
particularly in soils.

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